The trachea, colloquially called the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathinganimals with lungs. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea the cricoid cartilage attaches it to the larynx. The trachea is formed by a number of horseshoe-shaped rings, joined together vertically by ligaments over their substance and by the trachealis muscle at their ends. The epiglottis closes the opening to the larynx during swallowing.

The trachea begins to form in the second month of development, becoming longer and more fixed in its position over time. It is epithelium lined with column-shaped cells that have hair-like extensions called cilia, with scattered goblet cells that produce protective mucins. The trachea can be affected by inflammation or infection, usually as a result of a viral illness affecting other parts of the respiratory tract, such as the larynx and bronchi, called croup, that can result in a barking cough. Infection with bacteria usually affects the trachea only and can cause narrowing or even obstruction. As a major part of the respiratory tract, when obstructed the trachea prevents air entering the lungs and so a tracheostomy may be required if the trachea is obstructed. Additionally, during surgery if mechanical ventilation is required when a person is sedated, a tube is inserted into the trachea, called intubation.

An adult's trachea has an inner diameter of about 1.5 to 2 centimetres (0.59 to 0.79 in) and a length of about 10 to 11 centimetres (3.9 to 4.3 in); wider in males than females.[2] It begins at the bottom of the larynx, and ends at the carina, the point where the trachea branches into left and right main bronchi.[2] The trachea is surrounded by 16 - 20 rings of hyaline cartilage; these 'rings' are 4mm high in the adult, incomplete and C-shaped.[2] Ligaments connect the rings.[3] The trachealis muscle connects the ends of the incomplete rings and runs along the back wall of the trachea.[3]

The thyroid gland also lies on top of the trachea, and lies below the cricoid cartilage. The isthmus of the thyroid, which connects both wings, lies directly in front, whereas the wings lie on the front and stretch to the side.

The upper part of trachea receives and drains blood through the inferior thyroid arteries and veins;[2] the lower trachea receives blood from bronchial arteries.[3] Arteries that supply the trachea do so via small branches that supply the trachea from the sides. As the branches approach the wall of the trachea, they split into inferior and superior branches, which join with the branches of the arteries above and below; these then split into branches that supply the anterior and posterior parts of the trachea.[3] The inferior thyroid arteries arise just below the isthmus of the thyroid, which sits atop the trachea. These arteries join (anastamoses) with ascending branches of the bronchial arteries, which are direct branches from the aorta, to supply blood to the trachea.[2] The lymphatic vessels of the trachea drain into the pretracheal nodes that lie in front of the trachea, and paratracheal lymph nodes that lie beside it.[2]

In the fourth week of development of the human embryo as the respiratory bud grows, the trachea separates from the foregut through the formation of ridges which eventually separate the trachea from the oesophagus, the tracheoesophageal septum. This separates the future trachea from the oesophagus and divides the foregut tube into the laryngotracheal tube.[4] By the start of the fifth week, the left and right main bronchi have begin to form, initially as buds at the terminal end of the trachea.[4]

The trachea is no more than 4mm diameter during the first year of life, expanding to its adult diameter of approximately 2cm by late childhood.[2][3] The trachea is more circular and more vertical in children as compared with adults,[3] varies more in size, and also varies more in its position in relation to its surrounding structures.[2]

The trachea is surrounded by 16-20 rings of hyaline cartilage; these 'rings' are incomplete and C-shaped.[2] Two or more of the cartilages often unite, partially or completely, and they are sometimes bifurcated at their extremities. The rings are generally highly elastic but they may calcify with age.

Cross-section

Cross-section of the trachea, with pseudostratified ciliated columnar epithelium and goblet cells labelled

Tracheal intubation refers to the insertion of a tube down the trachea.[14] This procedure is commonly performed during surgery, in order to ensure a person receives enough oxygen when sedated. The catheter is connected to a machine that monitors the airflow, oxygenation and several other metrics. This is often one of the responsibilities of an anaesthetist during surgery.

In an emergency, or when tracheal intubation is deemed impossible, a tracheotomy is often performed to insert a tube for ventilation, usually when needed for particular types of surgery to be carried out so that the airway can be kept open. The provision of the opening via a tracheotomy is called a tracheostomy.[15] Another method procedure can be carried, in an emergency situation, and this is a cricothyrotomy.[16]

Tracheal agenesis[17] is a rare birth defect in which the trachea fails to develop. The defect is usually fatal though sometimes surgical intervention has been successful.

A tracheoesophageal fistula is a congenital defect in which the trachea and esophagus are abnormally connected (a fistula). This is because of abnormalities in the separation between the trachea and oesophagus during development.[4] This occurs in approximately 1 in 3000 births, and the most common abnormalities is a separation of the upper and lower ends of the oesophagus, with the upper end finishing in a closed pouch.[4] Other abnormalities may be associated with this, including cardiac abnormalities, or VACTERL syndrome.[4] Such fistulas may be detected before a baby is born because of excess amniotic fluid; after birth, they are often associated with pneumonitis and pneumonia because of aspiration of food contents.[4] Congenital fistulas are often treated by surgical repair.[10] In adults, fistulas may occur because of erosion into the trachea from nearby malignant tumours, which erode into both the trachea and the oesophagus. Initially, these often result in coughing from swallowed contents of the oesophagus that are aspirated through the trachea, often progressing to fatal pneumonia; unfortunately, there is rarely a curative treatment.[10] A tracheo-oesophageal puncture is a surgically created hole between the trachea and the esophagus in a person who has had their larynx removed. Air travels upwards from the surgical connection to the upper oesophagus and the pharynx, creating vibrations that create sound that can be used for speech. The purpose of the puncture is to restore a person's ability to speak after the vocal cords have been removed.[18]

Sometimes as an anatomical variation one or more of the tracheal rings are formed as complete rings, rather than horseshoe shaped rings. These O rings are smaller than the normal C-shaped rings and can cause narrowing (stenosis) of the trachea, resulting in breathing difficulties. An operation called a slide tracheoplasty can open up the rings and rejoin them as wider rings, shortening the length of the trachea.[19] Slide tracheoplasty is said to be the best option in treating tracheal stenosis.[20]

Mounier-Kuhn syndrome is a rare congenital disorder of an abnormally enlarged trachea, characterised by absent elastic fibres, smooth muscle thinning, and a tendency to get recurrent respiratory tract infections.[21]

From 2008, operations have experimentally replaced tracheas, with those grown from stem cells, or with synthetic substitutes, however this is regarded as experimental and there is no standardised method.[22] Difficulties with ensuring adequate blood supply to the replaced trachea is considered a major challenge to any replacement. Additionally, no evidence has been found to support the placement of stem cells taken from bone marrow on the trachea as a way of stimulating tissue regeneration, and such a method remains hypothetical.[22]

In birds, the trachea runs from the pharynx to the syrinx, from which the primary bronchi diverge. Swans have an unusually elongated trachea, part of which is coiled beneath the sternum; this may act as a resonator to amplify sound. In some birds, the tracheal rings are complete, and may even be ossified.[23]

In amphibians, the trachea is normally extremely short, and leads directly into the lungs, without clear primary bronchi. A longer trachea is, however, found in some long-necked salamanders, and in caecilians. While there are irregular cartilagenous nodules on the amphibian trachea, these do not form the rings found in amniotes.[23]

The only vertebrates to have lungs, but no trachea, are the lungfish and the Polypterus, in which the lungs arise directly from the pharynx.[23]

Tracheal system of dissected cockroach. The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm.

The tracheal system branches into progressively smaller tubes, here supplying the crop of the cockroach. Scale bar, 2 mm.

The word "trachea" is used to define a very different organ in invertebrates than in vertebrates. Insects have an open respiratory system made up of spiracles, tracheae, and tracheoles to transport metabolic gases to and from tissues.[24] The distribution of spiracles can vary greatly among the many orders of insects, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm. Diffusion of oxygen and carbon dioxide takes place across the walls of the smallest tubes, called tracheoles, which penetrate tissues and even indent individual cells.[25] Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their haemolymph.[26] This is one of the factors that may limit their size.

A tracheal tube may contain ridge-like circumferential rings of taenidia in various geometries such as loops or helices. Taenidia provide strength and flexibility to the trachea. In the head, thorax, or abdomen, tracheae may also be connected to air sacs. Many insects, such as grasshoppers and bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill, or function essentially as normal, via a plastron. Note that despite being internal, the tracheae of arthropods are lined with cuticular tissue and are shed during moulting (ecdysis).[25]

^Wasserthal, Lutz T. (1998). Chapter 25: The Open Hemolymph System of Holometabola and Its Relation to the Tracheal Space. In "Microscopic Anatomy of Invertebrates". Wiley-Liss, Inc. ISBN0-471-15955-7.

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